Its because its not nice. There is literally no reason to say in all chat that since you are insulting the enemy team. Say it in normal chat to your own team if you must. There is no excuse to do it in all and you will get banned for it since it is actually a very serious thing they look at.
Its no different than any type of competition or sport. Its one thing to be happy and revel in your victory, its another to mock the person or team that lost. Watch the olympics or some pro sport and see what happens when they once in a while go to far. They get fined pretty hard and get kicked out.